Production of plastic mesh structure

ABSTRACT

In order to produce high tensile strength mesh structures, starting material, which is at least 0.75 mm thick and which has a square or rectangular pattern of holes or depressions, is stretched either in one direction or in two directions at right angles. The uniaxially stretched structure has orientated strands which are connected by parallel bars, the orientation of the strands penetrating into the bars but the centers of the bars being thicker than the strands, all along the length of the bar center. The biaxially stretched structure has orientated strands which are interconnected by orientated junctions which have orientated crotches between the strands; the thinnest parts of the junctions are not less than 75% of the thickness of the strands, the thickest parts of the junctions are thicker than the thinnest parts and thicker than the strands, and the center of the junction is not the thinnest part of the junction.

This Application is a continuation-in-part of U.S. Ser. No. 82523 filedon Oct. 9th, 1979 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to plastics material integral meshstructures having mesh openings defined by a generally rectangular gridof substantially parallel, orientated strands and junctionstherebetween, and also to an intermediate structure which is producedwhen making the final structure and which has in itself commercial uses.

A problem in all integral mesh structures relates to the junctions. Thejunctions should be sufficiently strong without containing too muchplastics material.

U.K. patent specification No. 982 036 describes the production ofrectangular grid mesh structures by stretching a substantiallymonoplanar plastics material sheet comprising a pattern of holes whosecentres lie on a rectangular grid; the sheet is stretched biaxially intwo directions parallel to the axes of the grid. The junctions soproduced are not stretched and are thick and heavy.

U.K. patent specification No. 1 310 474 describes structures whosejunctions are strong enough for the light duty applications with whichthe Patent Specification is concerned but are not strong enough forheavy duty applications. FIG. 5 of the Patent Specification shows twopairs of strands running into the junction, and the junction is formedof crotch filaments, a centre filament and thin webs. The webs may beabout half the thickness of the strands. It is found that on rupture,the structure very often breaks at the junction, a split starting in aweb, which acts as a tear starter, and running along the strands. Thealternative structure illustrated in FIG. 4 is very similar except thatthe centre filament is absent, and its behaviour is similar.

U.K. patent specification No. 1 544 608 describes structures which arethin and flat, the junctions being no thicker than the strands. It isbelieved that a flat junction is not the strongest junction as, at thevery centre of the junction, there is an area of random moleculararrangement which has less resistance to rupture than the molecularlyorientated areas which surround it; the surrounding areas have the samethickness as, and are therefore stronger than, the centre area.

Definitions

The term "rectangular" includes square.

The term "orientated" means molecularly orientated.

The terms "rows" and "columns" are used for convenience to denote theaxes of the rectangular grid. Unless specified otherwise, it does notmatter if the rows extend in the machine direction (MD) or in thetransverse direction (TD).

The terms "thick", "thin", "thickness", "deep", "depth", and "shallow"refer to the dimension normal to the plane of the starting material ormesh structure and the terms "wide", "narrow", and "width" refer to theappropriate dimension in the plane of the starting material or meshstructure.

The thickness of the starting material or mesh structure is the distancebetween the extreme faces of the starting material or mesh structure.

The thickness or depth of a strand is the thickness of the strandcross-section, but ignoring raised edges. Particularly if the originalholes or depressions have no radiussing where they issue at the faces ofthe starting material, the strands will have a "pin-cushion"cross-section, with raised edges and lower centres; the thickness ordepth will be as measured inwardly of the raised edges.

The notional junction zones of the starting material are notional zonesdefined by the intersection of the notional parallel-sided zone whichlies between and is tangential to two columns of holes or depressionsand the notional parallel-sided zone which lies between and istangential to two rows of holes or depressions.

The term "tangential" includes touching, but not intersecting, astraight sided hole, depression or opening or touching a corner of ahole, depression or opening.

The depressions are not necessarily formed by the application ofpressure.

Stretch ratios are either given overall or "on the strands". If they aregiven on the strands, they are measured by measuring the distance movedby the respective ends of the openings on either side of the strand. Forthe second stretch, the ratios are measured by comparing the stretchedlengths with the original starting material and not with the material asafter the first stretch. The ratios are as measured after relaxation.

THE INVENTION

The invention provides both methods and structures manufactured by thosemethods.

In the method of one embodiment of this invention, uniaxial orientationis used and the structures so produced are principally uniaxiallyorientated. In the method of another embodiment, biaxial orientation isused, and the structures so produced are biaxially orientated.

The starting material, when biaxially stretched in accordance with oneembodiment of this invention, produces junctions between the strandswhich are not flat but do not exhibit any excessive thinning, and thereare no filaments within the junctions. The whole junction has a minimumthickness which is not less than 75% of the thickness of the mid-pointof any of the strands passing into the junction. Each junction is asolid junction as opposed to an openwork junction formed by a frameworkof sub-filaments and film or by an orientated thin film area boundedabout its perimeter by orientated filaments. The junctions have acentral zone which is thicker than orientated lateral zones on at leasttwo opposite sides thereof which central zone can if desired includesome unorientated material (or there may be two small, spacedunorientated zones on either side of the centre of the junction). Thisunorientated or random orientated central zone is thicker than thestrands and thus can have sufficient strength to prevent rupturingoccurring in the centre of the junction. The junctions maintain a shapewhich provides good stress transmission paths and permits the junctionto withstand high forces between either pair of aligned strands orbetween two strands initially at 90° to each other. The structure isstrong enough to be used for instance as stock fencing, provided thestrands are of sufficiently heavy gauge, or relatively stronglight-weight structures can be provided for instance for oliveharvesting.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be further described, by way of example, withreference to the accompanying drawings, in which:

FIGS. 1, 2 and 3 show three stages in a method in accordance with theinvention;

FIGS. 4a, 4b and 4c are sections along the corresponding lines shown inFIG. 2;

FIG. 4d is a section corresponding to that of FIG. 4c, but shows avariation;

FIGS. 5 to 9 show the junctions in five structures produced by themethod;

FIGS. 9a and 9b are sections taken along the lines IXA--IXA and IXB--IXBof FIG. 8;

FIG. 10 shows various shapes of holes or depressions that can be used inthe starting material;

FIGS. 11 and 11a are schematic elevations of two different plants formaking biaxially stretched structures in accordance with the invention;

FIG. 12 is a perspective view of a stabilizing or retaining structure inaccordance with the invention;

FIG. 13 is a vertical section through a wall, earth retained thereby andstructures as in FIG. 12;

FIG. 14 is a vertical section through an embankment stabilised inaccordance with the invention;

FIGS. 15 to 19 are schematic representations of further uses of theinvention;

FIG. 20 corresponds to FIG. 2, but shows a slightly different structure;

FIG. 21 shows the junction of a structure made from the structure ofFIG. 20,

FIGS. 21a to 21d are schematic sections taken along the linesXXIA--XXIA, XXIB--XXIB and XXIC--XXIC of FIG. 21, not to scale;

FIG. 21d is a schematic developed view of the crotch edge, generallylooking in the direction of the arrow XXID in FIG. 21, not to scale; and

FIGS. 22 to 22d correspond to FIGS. 21 to 21d, but show a slightlydifferent structure.

In FIGS. 2, 5 to 9, 12, 21 and 22, the hatching indicates a steep slope(increase in thickness), the hatching lines extending up the slope.

Uniaxially Stretched Structures

Looking at FIG. 1, the starting material is a sheet of plastics material11 having planar faces and in which are formed circular holes ordepressions 12. The "holes" 12 need not pass right through the sheet andcan be depressions in one or both sides of the sheet, leaving acontinuous membrane, preferably on the median plane of the sheet. FIG. 1shows what is termed herein a notional junction zone 13, namely thenotional zone defined by the intersection of the notional parallel-sidedzone 14 which lies between and is tangential to two columns of holes ordepressions 12, and the notional parallel-sided zone 15 which liesbetween and is tangential to two rows of holes or depressions 12. FIG. 1also shows truth lines 14', 15' which would not be used in commercialoperation but can be scribed or drawn on the plastics material to showwhat is happening.

When the sheet 11 is drawn in the vertical direction (looking at FIG. 1)the structure of FIG. 2 is formed because the zones 16 (FIG. 1) arestretched and orientated into strands 17. The stretching is performed tosuch an extent, for instance to a ratio of 7:1 on the strands, that theoutermost portions of the notional junction zones 13 are orientated andstretched out to form the end portions of the strands 17, which mergesmoothly with the remainder of the strand (see FIG. 4c); the orientationcan pass right through or nearly through the centre of each notionaljunction zone 13. A notional point 18 (FIG. 1), which on the startingsheet 11 lay on the notional straight line 19 which is parallel to andtangential to the rows of holes or depressions 12, has moved into thecorresponding strand (FIG. 2) so that it is spaced from thecorresponding notional straight line 19' by a substantial distance x(FIGS. 2 and 4c). This is also illustrated by the truth lines 15' inFIG. 2. The distance x is preferably not less than 25% of the thicknessof the mid-point of the strands 17 and more preferably not less thansuch thickness.

In effect, the notional parallel-sided zones 15 form bars runninghorizontally as seen looking at FIG. 2, each comprising a succession ofalternate zones, namely first zones 20 between and interconnecting theends of aligned strands 17 and second zones 21 between the first zones20. The second zones 21 are not substantially orientated, and it will beseen from FIG. 4a that they still maintain the original thickness of thesheet 11. They have flat outer faces in FIG. 4a, and may also have flatouter faces in FIG. 4b. However, the first zones 20 have been orientated(see the undulating top and bottom surfaces in FIG. 4a); the orientationcan pass right through the first zones 20 in the direction of strands17, forming a trough in the horizontal bar, as can be seen in FIG. 4a,all of the first zone 20 being orientated in the direction of thestrands 17. The centre of each first zone 20 (corresponding to themid-point of the notional junction zone 13) is substantially thicker andless orientated than the strands 17 (see FIG. 4c), and can have athickness ranging from just somewhat thicker than the strands 17 to thethickness of the starting material 11. If all the first zone 20 isorientated, its central third may be stretched to a ratio of at least1.5:1. If the central part of the first zone 20 is not stretched, thelength of the part that has not been stretched may be for example up tofive times its thickness if the bars are wide or not greater than itsthickness. In the structure shown in FIG. 4c, there is a gradualincrease in thickness from the point 18 to the centre of each first zone20. At 23 in FIG. 2, the material of the notional junction zone 13 isdrawn out, forming a re-entrant on either side of the first zone 20.

FIG. 4d illustrates a variant that can occur. The orientation has passedright through the first zone 20 but there has only been a slightthinning in the centre of the zone 20, there being sharper steps down tothe thickness of the strands 17 near the edges of the zone 20.

For uniaxially stretched structures, the starting material can be of anysuitable thickness from 0.75 mm upwards; better structures can beproduced with starting material thickness of at least 1 mm.

The distance between adjacent holes or depressions 12 in the startingmaterial 11 may be greater than the thickness of the starting material11 at the same point.

The uniaxially stretched structures can find many uses, as set forthbelow. By having the orientation passing into the first zones 20, thereis a saving of plastics material; by having the orientation passingright through the first zones 20, there is a degree of orientationconnected aligned strands 17 and a reduction in the amount of yieldwhich would occur within the bar under tension in the vertical direction(as seen in FIG. 2); and by having the centre of each notional junctionzone 13 substantially less orientated, there is a reduced danger ofsplitting when flexing the bars.

Biaxially Stretched Structures

The structure of FIG. 2 can alternatively be subjected to a secondstretching operation, in the horizontal direction as seen looking atFIG. 2. The effect of this second stretching operation is to draw outthe zones 24 indicated in FIG. 1, which correspond to the second zones21 shown in FIG. 2, forming further strands 25 as shown in FIG. 3. Atthe same time, it is found that if no tension is applied in the verticaldirection, the length of the openings in the direction of the firststretching operation is reduced, possibly by up to 33%, and the endportions of the strands 17 are either partly or fully drawn into thejunctions and even drawn out in the direction of the second stretchingoperation to form end portions of the strands 25 shown in FIG. 3, thestructure shortening correspondingly in the first direction. This isillustrated by the truth lines 14' in FIG. 3. Thus the outermostportions of the original notional junction zones 13, at the end of thefirst stretching operation, can have an orientation which is in thedirection of the first stretching, and, at the end of the secondstretching operation, can have a predominant orientation which is in thedirection of the second stretching or can have approximately equalorientation in each of these two directions. The amount of this effectdepends upon the overall stretch ratios in the two stretchingoperations, which are discussed further below.

It is believed that if a degree of orientation passes right through orat least nearly right through the notional junction zone 13, a betterjunction can be produced in the final product. However it has been foundthat orientation need not even pass nearly right through the zone 13.

FIGS. 5 to 9, 21 and 22 illustrate some examples of junctions 26 whichare formed between the strands 17, 25. As indicated above, the firststretch was up and down the sheet of drawings and the second stretch wasacross the sheet. It will be seen that each of these junctions 26 is ofgenerally lozenge or lenticular shape (particularly in FIGS. 7 to 9)with its major axis or maximum dimension aligned with the strands 25formed in the second stretching operation and larger or much larger thanits minor axis or minimum dimension, which is aligned with the strands17. The sides of the junction 26 form curved crotches and merge verygradually with the sides of the strands 25 but merge relatively abruptlywith the sides of the strands 17. The size of the junction 26 is muchlarger than that of the notional intersection zone 26' which would beformed by the intersection of the strands 17, 25 (see FIG. 5).

Each junction 26 is substantially symmetrical about a plane parallel tothe plane of the mesh structure, namely the median plane of the meshstructure, but each junction 26 is not flat, having a specific contour.The minimum thickness of each junction 26 is not less than 75% of thethickness at the mid-point of any of the strands 17, 25; it is believedthat as the minimum thickness decreases below the thickness of themid-point of the thickest strand 17 or 25, down to 90% or 80% of thevalue, or below, the strength of the junction decreases. The maximumthickness of the junction is substantially greater than the minimumthickness and substantially greater than the thickness of the mid-pointof any of the strands 17, 25.

It is normal practice to measure the thickness of a mesh strand at itsmid-point and this is the point one would naturally choose. However, ithas been observed that, particularly if the original holes ordepressions were circular, the mid-point of a strand may not be itsthinnest point.

Each junction 26 has a central zone 27 which is thicker than orientatedlateral zones 28, 28' on at least two opposite sides thereof andnormally thicker than the mid-points of at least two of the strands 17,25. Thus, in FIGS. 5 to 9, 21 and 22, there is a substantial increase inthickness as one passes through the junction 26 from one strand 17 tothe aligned strand 17. If the orientation did not pass right through thefirst zones 20 during the first stretch, the central zone 27 would tendto be thicker. In general, the central zone 27 will be substantiallyless orientated than the lateral zones 28, 28' and the centre part ofthe central zone 27 may not even be orientated, though the major part ofthe junction should be orientated. In the worse case, only 70% of theplan area of the junction would be orientated. There is a high degree oforientation in the direction running around the crotches betweenadjacent strands 17, 25.

It will be seen that the stretching is effected until the edge zones ofthe crotches which extend between respective adjacent pairs of strands17, 25 passing into the junction 26 are orientated in the directionrunning around the respective crotch, and the stretching is terminatedwhile each junction 26 has a minimum thickness not less than 75% of thethickness of the mid-point of any of the strands 17, 25 passing into thejunction, has a maximum thickness substantially greater than its minimumthickness and substantially greater than the thickness of the midpointof any of the strands passing into the junction 26, and has a centralzone 27 which is thicker than orientated zones on two opposite sidesthereof.

In practice, it is found that a useful way of determining the processparameters for new structures is to stretch material in the laboratory,for the first stretch observing by eye the penetration of theorientation into the zones 20, and for the second stretch observing byeye the drawing out of the zones 21 until the orientation passes fromthe ends of the strands 25 so formed around the crotches to the strands17, the subsequent thinning down of the crotches, and the changingcontour of the junction as the orientation progresses from the ends ofthe strands 25 into the junction. In this manner, with a given startingmaterial, the desired stretch ratios can be determined for the first andsecond stretches--the stretch ratios should be taken say 5% beyond thosedesired to allow for relaxation.

As noted above, the junction 26 of FIGS. 7 to 9 has its major axisaligned with the direction of the second stretching operation, i.e. ofthe strands 25, and the structure will have greater strength in thisdirection if the cross-section and spacings of the strands 17 and 26 areequal. The ratio of the major to minor dimensions of the junction 26 canbe varied and a more balanced orientation and shape produced by carefulselection of the stretch ratios in the two stretching operations.Although the ratio applied during the second stretching operation can begreater than that applied during the first stretch operation, some ofthe second stretching is involved in pulling out the junctions andshortening the strands 17. Increasing the stretch ratio in the secondstretching direction increases the strength in that direction butreduces the strength in the other direction.

It will be seen that in all the junctions 26 except those of FIGS. 6 and9, there is no steep slope (rapid increase in thickness) or abruptchange in thickness when one looks parallel to the plane of thestructure at the profile of the edge of any crotch between the strands17 and 25. Furthermore there is a marginal zone 28" extending aroundeach crotch which is relatively flat and is of significant width, thisfeature being most marked in the structures of FIGS. 5, 21 and 22. It isbelieved advantageous to have such crotch edges and such crotch marginalzones 28" as the amount of plastics material in the junction is reducedwithout gross loss of strength. In the best structures of this type, aslope angle of up to 5° (10° included angle for both sides) can beformed, as measured on a developed view of the crotch edge, thoughsignificant savings in plastics materials are possible up to angles of35° (70° included angle). As mentioned above, the centre part of thecentral zone 27 may not be orientated i.e. having the thickness of theoriginal starting material. This in particular is acceptable if theedges and marginal zones referred to above are present. Such edges andmarginal zones 28" can be obtained with the procedure for making a FIG.21 or 22 junction, as described below.

Alternatively, if there is a steep slope on the crotch edge or no markedmarginal zone (as in FIGS. 6 and 9), it is preferred that the whole ofthe junction be substantially orientated and be substantially thinnerthan the original starting material.

In FIG. 5, the second zones 21 (see FIG. 2) have stretched before thefirst zones 20, and the first zones 20 have not been fully stretched (oreven a small centre zone of unorientated material was left in each ofthe zones 20), leaving the central zone 27 of the junction 26 in theform of a lump. The maximum thickness of the junction 26 is in the lumpor central zone 27, and the lump is surrounded by thinner, orientatedzones with which the lump merges. The lateral zones 28,28' indicated inFIG. 5 are orientated and can have a thickness which is slightly greaterthan that of any of the strands 17 or 25, where they pass into thejunction 26, and roughly equal to or slightly greater than the thicknessof the mid-point of the strands 17 and 25. As mentioned above, there areflat marginal zones 28" around each crotch. The structure can haveapproximately the same strength along each axis if the cross-section andspacings of the strands 17 and 25 are equal. The formation of the typeof junction 26 shown in FIG. 5 is facilitated by not allowing thematerial to shorten in the second stretching direction when carrying outthe first stretching operation, allowing the orientation to pass wellinto but not right through the first zones 20. The formation is furtherfacilitated by having some restraint in the first stretching directionwhen carrying out the second stretching operation.

FIGS. 21 to 21d illustrate a junction 26 which is very similar to thatof FIG. 5, but has been produced in a continuous production plant asdescribed below with reference to FIG. 11 and Example 12. FIG. 20 showsthe intermediate or uniaxially stretched structure. There was 5.5%sideways contraction during the first stretch but no lengthwisecontraction during the second stretch. The central zone of lump 27 issmaller in plan view and also somewhat thinner, and the surroundingzones 28,28' and 28" of highly orientated material are wider.

As can be seen in FIG. 21d, the edges of the crotches have a slightthickening (maximum slope 2°, maximum included angle 4°), but there isno sharp or abrupt increase in thickness. As can be seen in FIG. 21c,the marginal zones 28" of the crotches are flat and nearly parallelfaced, the central zone 27 then rising sharply as one approaches thecentre of the junction. The central zone 27 is more quadrilateral thanthat of FIG. 5, and while its N and S sides are convex, its E and Wsides are concave, having slight reentrants on the axes of the strands25. The top of the central zone 27 is nearly flat, but has a slightconvexity, the thickest point being in the very centre.

FIGS. 22 to 22d illustrate a similar junction to that of FIGS. 21 to21d, but produced on laboratory equipment as described below in Example13. The marginal crotch zones 28" are not so wide (maximum slope 2°,maximum included angle 4°), as for FIG. 21; the central zone 27 risesmore steeply, has almost rectilinear N and S sides and less markedreentrants on the E and W sides. Again, the top of the central zone 27is nearly flat, but in this junction, the highest points of the centralzone 27 are on the E-W axis, just adjacent the reentrants, though thethickness there is only about 2-3% greater than that at the very centre.

FIG. 6 illustrates a junction 26, generally as that of FIG. 5 wouldappear on further stretching in the second direction. The central zone27 is rather more rectangular in appearance. The crotches are stillcurved smoothly, with orientation in their edge zones running around thecrotches, but are bulged outwardly at the corners of the central zone27. The overall shape of the central zone 27 is not dissimilar to thatof FIG. 21, but it is relatively larger and its edges (which slope down)extend as far as, and form part of, the edges of the crotches, giving asteep rise as one follows round the edges of the crotches.

FIG. 7 illustrates a junction 26 generally as that of FIG. 6 wouldappear on further stretching in the second direction. The raised centralzone 27 is long-shaped and aligned with the strands 25, and extends intozones or lumps 29 at each end, which lumps 29 are thicker than thecentral zone 27 and adjacent the ends of the strands 25, the wholeforming a dog-bone-like shape as can be seen in the Figure There is areentrant at end E and W end of the central zone 27 and lumps 29, on theaxes of the strands 25.

FIG. 8 illustrates a junction 26 generally as that of FIG. 7 wouldappear on further stretching in the second direction. The central zone27 extends into lumps 30, the whole forming a dumb-bell-like shape butgenerally similar to the dog-bone of FIG. 7. However, the E and W endsof the central zone 27 and lumps 30 slope down gradually into thestrands 25.

FIG. 9 illustrates a junction 26 generally as that of FIG. 8 wouldappear on further stretching in the second direction. The central zone27 is long-shaped and merges smoothly with each of the strands 25 withno rapid decrease in thickness, though there is a slight thickening inthe zones 31. Two sections through the junctions 26 are shown in FIGS.9a and 9b. The edges of the zones 31 extend as far as, and form part of,the edges of the crotches, giving a steep rise as one follows round theedges of the crotches.

Production of the junctions 26 described above depends upon the shapesand spacings of the holes or depressions, stretching conditions such asthe temperature, and the plastics material. The details given below aregiven for guidance and not to limit the invention.

There is a tendency for junctions of the type shown in U.K. patentspecification No. 1 310 474 to be formed if the sheet thicknesses arebelow 1.5 mm, particularly if the w:d ratio (the ratio of the distance wbetween holes or depressions of adjacent columns or rows in the startingsheet to the thickness d of the sheet) is too high; this tendencyincreases as the sheet thickness decreases below 1 mm and particularlyas the sheet thicknesses decrease into the range of 0.75 mm down to 0.5mm. This tendency can be reduced by avoiding any raised edges around theholes, e.g. caused by embossing, or by reducing the w:d ratio. However,a preferred lower limit for the thickness of the starting material is 1mm, for which it has been found that the thickest zone of a junction 26can be down to about 0.7 mm thick; for 0.75 mm thick starting material,the corresponding junction thickness would be about 0.55 mm.

More generally, it is believed that the behaviour of the material altersat smaller thicknesses because the sizes of the molecules themselvesbecome more relevant. It is believed that one will not necessarilyachieve a structure like that of the invention using a starting materialwhich, compared to that of any of the Examples below, is scaled down(i.e. in thickness, hole size and pitch in each direction), e.g. tothicknesses of 0.5 mm and below.

It is preferred to use stretching temperatures which are belowmanufacturers' recommended stretching temperatures, e.g. 97° C. for HDPE(high density polyethylene) instead of just below 126° C.

In the first stretching operation, the orientation may not pass throughthe notional junction zones 13 (FIG. 1), if this is desired, or may noteven pass sufficiently far into the zones 13. This tendency can beavoided or reduced if desired by decreasing the distance between theholes or depressions in the first stretching direction (decreasing thew:d ratio), decreasing the distance between the holes or depressions inthe second stretching direction or decreasing the radius of the cornersof the holes or depressions.

Though the limits may be exceeded by adjustment of other parameters, ithas been found that for satisfactory orientation for uniaxiallystretched structures, the w:d ratio should be around 6.35 or below andfor biaxially stretched structures around 4.23 or below.

The stretch ratios employed will depend on the actual plastics materialused and on the dimensions of the starting material, but as a generalindication, for biaxially stretched structures with satisfactoryorientation of the junctions without excessive material in thejunctions, an area stretch ratio (ratio of final, relaxed area tooriginal area) of at least about 13:1 should be produced.

If there is to be substantial restraint in the first stretchingdirection during the second stretching operation, care should be takento avoid pulling the zones 20 (FIGS. 2 and 20) right out; pulling thezones 20 right out forms thin zones in the junctions. It is believedthat if the zones 21 orientate before the zones 20 in the secondstretch, the unwanted junction will not be produced, but it is notnecessarily undesirable that the zones 20 should orientate first,particularly if there is no restraint during the second stretch. If theunwanted junctions are produced, a test piece should be made in whichthe orientation penetrates less through the zones 20 in the firststretch; this can be done during the first stretch by decreasing thestretch ratio and/or increasing the restraint in what will be the secondstretch direction. More specifically, if there is to be substantiallyrestraint during the second stretch, the orientation during the firststretch is preferably not taken right through the zones 20, though theorientation should pass beyond the notional tangent line 19 (FIG. 1) or19' (FIGS. 2 and 20) to make a junction that does not contain too muchplastics material. Difficulties may be encountered if the penetration(distance y in FIGS. 2 and 20) is greater than 30% of the distancebetween the notional tangent lines 19', 19" (defining a parallel-sidedzone or the bar width), though the structure shown in FIG. 2 may besatisfactory if there is not excessive restraint during the secondstretch. The preferred percentage for trouble-free operation is 25% andbelow, 20% and below being a more acceptable value.

In general, decreasing the w:d ratio increases the tear resistance.

General

The starting material can be of any suitable thickness from 0.75 mmupwards: and in sheet form or tubular. The preferred material isstrictly uniplanar, by which is meant that, ignoring any membrane (whichmay not lie on the median plane), all zones of the starting material aresymmetrical about the median plane of the starting material. However,insubstantial departures from uniplanarity are not excluded. The holes(or depressions if suitable) can be formed by punching or by formingthem as the starting material itself is formed, obturating a slit diefor instance generally as in French patent specification No. 368 393. Ingeneral, it is preferred to avoid any substantial protruberance aroundthe periphery of the holes or depressions, particularly when producingbiaxially stretched structures; thus the zones 21 preferably have flattop and bottom faces, as illustrated in FIGS. 4c and 4d, and this isbelieved to reduce any tendency for thin spots to form in the junctionsof the biaxially stretched structures. If depressions are formed, themembrane closing the depressions can be ruptured during stretching andthe residual film-like material removed.

The starting material is preferably not substantially orientated, thoughmelt flow orientation can be present.

The starting material can be any suitable thermoplastics material, suchas for instance HDPE, low density polyethylene, polypropylene (pp),copolymers of HDPE and polypropylene, and polyamides. The startingmaterial can have a skin on each face containing an ultra-violetstabiliser--the greater the width:depth ratio of the orientated strandsin the product the more effective the ultra-violet stabilisation as theunstabilised sides of the strands and junctions form a smallerproportion of the total surface area. To enable the mesh structure to beused for laminating, whether to one or more like mesh structures or toone or more different materials such as fabric or film, the startingmaterial may have a special layer on one or both faces. This layer maybe of a substance such as low density polyethylene or ethylene vinylacetate which melts or becomes tacky at a temperature at which the maincomponent of the structure would not de-orientate. The layer or layerscould be applied by coextrusion coating.

After stretching, the structures can be annealed in a manner well known.

FIG. 10 shows various shapes for the holes or depressions. For producinguniaxially or biaxially stretched structures, the grid on which thecentres lie can be square or rectangular.

Depending somewhat on the shape of the holes, in general the area of theholes or depressions is preferably less than 50% of the plan view areaof the starting material, and more preferably less than 25%.

As related to the cross-sectioned area of the mid-points of the strands17 or 25, the biaxially-stretched polypropylene structures of theinvention can have tensile strengths at break, in each strand direction,of greater than 400 Newtons/mm² ; breakage normally takes place at ornear the mid-points of the strands, but if it does not do so, thetensile force is still measured at break. Less robust polypropylenestructures, while still being in accordance with the invention, can havea strength, in each direction, or an average of the strengths in each ofthe two directions, down to 200 or 300 N/mm².

These strengths should also be related to the weight of plasticsmaterial in the junction or per square meter of plan view area of thestructure, as such weights can be significantly lower than those in forinstance U.K. patent specification No. 982 036. The strengths must alsobe related to the plastics material used--for instance HDPE is generallyweaker than polypropylene.

As related to the cross-sectional area of the mid-points of the strands17, the uniaxially-stretched polypropylene structures of the inventioncan have tensile strengths at break, at right angles to the bars, ofgreater than 400 or 500 Newtons/mm². Less robust polypropylenestructures, while still being in accordance with the invention, can havetensile strengths of 300 N/mm² or even lower. These strengths should berelated to the weight of plastics material in the bars or unit weight ofthe structure, though the bar widths will be chosen in accordance withthe specific use of the structures. As above, different strengths mustbe expected for different plastics materials.

Plant

The plant is shown schematically in FIG. 11, but the units themselvesare conventional units.

In the plant of FIG. 11, there is a let-off unit 41 supporting a roll 42of unperforated starting material which passes through the plant alongthe path indicated by the dashed lines and arrows. The starting materialpasses through a sheet flattening unit 43, a perforator 44, a transversedirection (TD) orientation (stretching) machine 45 in the form of a clipstenter, a machine direction (MD) orientation (stretching) machine 46,and is wound up on a wind-up unit 47. Hot air heating is employed ineach machine 45, 46. The material is not cooled down substantiallybetween the first and second stretches. After the second stretch, thereis a small amount of hot MD relaxation, and the structure is then cooledwhile held in its slightly relaxed configuration. In the secondorientation machine 46, the amount of transverse contraction depends onthe distance between the nips; if the distance is not too short, sometransverse contraction of the mesh structure will occur. Ifsubstantially no transverse contraction should occur, the nips should bebrought very close together, though this gives problems due tointerference of the rolls and drives.

In the plant of FIG. 11a, the TD orientation machine 45 comes after theMD orientation machine 46. In this case, with relatively short nips inthe MD orientation machine, significantly less transverse contractionoccurs than in the plant of FIG. 11. In the TD orientation machine 45 MDtension can be maintained so that there is no bow-back of the meshstructure as it is stretched transversely, so that the structure doesnot contract (shorten) substantially in the machine direction.

The first stretching operation can be carried out in the transversedirection or in the machine direction in a continuous plant. It iseasier to have full restraint during TD stretching and to allowcontraction during MD stretching.

EXAMPLES

Tables 1 and 2 set out the procedure and results, respectively for 14different Examples. All dimensions are in mm. "--" means that the valuewas not recorded. Stretch ratios are overall. For the w/d ratios inTable 1, w was measured at right angles to the first stretch direction.In Table 1, the hole size is the hole diameter (or width in the case ofExample 3). In Table 2, all the columns record thicknesses unlessotherwise indicated. y% is the percentage y is of the bar width. Theweight per m² is per square meter of plan view (including openings).

In Examples 1 to 10, there was no restriction of the material in thedirection at right angles to the stretching direction, both in the firststretch and in the second stretch. In Example 11, there was somerestriction, though not complete restriction, in the direction at rightangles to the stretching direction during the second stretch, the firststretch being unrestricted. In Example 13, there was full restraint inboth stretching directions. The restraints in Examples 12 and 14 areindicated below.

In the Examples, there were variations across the sample due to smallchanges in thickness of the starting material and other reasons, but theresults given are believed to be representative of the structuresobtained. Examples 1 to 11 and 13 were prepared on laboratory testingapparatus. Examples 12 and 14 were taken off production runs, and therewas somewhat more irregularity.

The process conditions for Example 12 were as follows:

Plant: as FIG. 11a;

Starting material width: 824 mm, including 20 mm margin on each edge(distance of side most edge of hole to edge of starting material). Thismargin can later be trimmed, if desired;

Initial speed of advance: 5 meters/minute;

Rate of stretch, first (MD) stretch: over a nip length of 300 mm;

Rate of stretch, second (TD) stretch; over an MD length of 8 meters;

Relaxation (TD only): 5%

During the MD stretch, 4.5% transverse contraction occurred, althoughthere was considerable restraint. No MD contraction occurred during theTD stretch.

The process conditions for Example 14 were as follows:

Plant: as FIG. 11;

Starting material width: as for Example 12;

Initial speed of advance: 5 meters/minute;

Rate of stretch, first (TD) stretch: over a MD length of 8 meters;

Rate of stretch, second (MD) stretch: over a nip length of 300 mm;

Relaxation (MD only): 5%.

No MD contraction occurred during the TD stretch. During the TD stretch25% transverse contraction occurred, which is virtually as though therewas no transverse restraint.

It was observed that in Examples 1 to 11 and 13, stretching beganrandomly throughout the material in the zones corresponding to theeventual mid-points of the strands. This could not occur during the MDstretching in Examples 12 and 14, which are of a continuous process.

                                      TABLE 1                                     __________________________________________________________________________                            Pitch                                                                              Pitch in                                                                 in 1st                                                                             other                                                                              1st 2nd Area                                        Starting                                                                           Hole                                                                             Hole    stretch                                                                            stretch                                                                            stretch                                                                           stretch                                                                           stretch                                                                           Temp                            No.                                                                              Material                                                                           thickness                                                                          size                                                                             shape                                                                             w/d direction                                                                          direction                                                                          ratio                                                                             ratio                                                                             ratio                                                                             °C.                      __________________________________________________________________________    1  HDPE 4.5  6.35                                                                             cir-                                                                               1.056                                                                            12.7 11.1 4.5:1                                                                             0   --  97                                              cular             relaxed                                                                       to                                                                            4.25:1                                      2  "    "    "  cir-                                                                              4.23                                                                              19.05                                                                              25.4 3:1 0   --  "                                               cular                                                         3  "    1.5  12.7                                                                             square,                                                                           2.12                                                                              25.4 15.88                                                                              4.5 0   --  "                                               radius-                                                                       sed                                                                           corners                                                       4  "    1    6.35                                                                             cir-                                                                              6.35                                                                              12.7 12.7 6.1 0   --  "                                               cular                                                         5  "    4.5  "  cir-                                                                              1.41                                                                              "    "    4:1 3.5:1                                                                             14:1                                                                              "                                               cular                                                         6  "    "    12.7                                                                             cir-                                                                              2.11                                                                              22.2 22.2 "   3.8:1                                                                             15:1                                                                              "                                               cular                                                         7  "    "    6.35                                                                             cir-                                                                              3.52                                                                              12.7 "    "   4.5:1                                                                             18:1                                                                              "                                               cular                                                         8  "    "    12.7                                                                             cir-                                                                              2.11                                                                              22.2 22.2 "   5.5:1                                                                             22:1                                                                              "                                               cular                                                         9  "    1.5  6.35                                                                             cir-                                                                              4.23                                                                              12.7 12.7 5:1 5:1 25:1                                                                              110                                             cular                                                         10 "    "    "  cir-                                                                              4.23                                                                              "    "    "   "   25:1                                                                              120                                             cular                                                         11 "    4.5  3.18                                                                             cir-                                                                              0.71                                                                              6.35 6.35 3.5:1                                                                             3.75:1                                                                            13:1                                                                              97                                              cular                                                         12 P P  3    6.35                                                                             cir-                                                                              2.12                                                                              12.7 12.7 4.25:1                                                                            5.9:1                                                                             25:1                                                                              98                                              cular                                                         13 "    "    "  cir-                                                                              "   "    "    4.5:1                                                                             4.25:1                                                                            19:1                                                                              99                                              cular                                                         14 HDPE "    "  cir-                                                                              "   "    "    3.75:1                                                                            3.62:1                                                                            14:1                                                                              98                                              cular                                                         __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________              Mid-       Uniax                            Biax                                                                             Biax                        Mid-                                                                             point      gm. Uniax                                                                              Mid- Mid-                                                                              Mid-        29,                                                                              gm.                                                                              tensile              Most   point                                                                            strand     wt. tensile                                                                            point                                                                              point                                                                             point       30 wt.                                                                              strength             similar                                                                              zone                                                                             17,        per strength                                                                           strand                                                                             strand                                                                            zone                                                                             Zone                                                                             Zone                                                                             Zone                                                                             or per                                                                              N/mm.sup.2           No.                                                                              FIG.                                                                              20 uniax                                                                             x y % y                                                                              m.sup.2                                                                           N/mm.sup.2                                                                         17, biax                                                                           25  27 28 28'                                                                              28"                                                                              31 m.sup.2                                                                          N-S                                                                              E-W               __________________________________________________________________________    1  2   4.23                                                                             1.35                                                                              4 --                                                                              -- --  --   --   --  -- -- -- -- -- -- -- --                2  *   4.5                                                                              1.39                                                                              --                                                                              --                                                                              -- --  --   --   --  -- -- -- -- -- -- -- --                3  *   1.5                                                                              0.51                                                                              --                                                                              --                                                                              -- --  --   --   --  -- -- -- -- -- -- -- --                4  *   1  0.23                                                                              --                                                                              --                                                                              -- --  --   --   --  -- -- -- -- -- -- -- --                5  6   -- --  --                                                                              --                                                                              -- --  --   1.37 1.61                                                                              4.33                                                                             1.63                                                                             2.43                                                                             -- -- -- -- --                6  7   -- --  --                                                                              --                                                                              -- --  --   1.3  1.8 2.7                                                                              1.4                                                                              -- -- 3.8                                                                              -- -- --                7  8   -- --  --                                                                              --                                                                              -- --  --   1.5  2   2  1.5                                                                              -- -- 3.2                                                                              -- -- --                8  9   -- --  --                                                                              --                                                                              -- --  --   1.2  1.8 1.6                                                                              1.36                                                                             -- -- 2.4                                                                              -- -- --                9  9   1.3                                                                              0.33                                                                              --                                                                              --                                                                              -- --  --   0.35 0.43                                                                              0.58                                                                             0.37                                                                             -- -- 1.15                                                                             -- -- --                10 9   1.33                                                                             0.33                                                                              --                                                                              --                                                                              -- --  --   0.4  0.38                                                                              0.62                                                                             0.37                                                                             -- -- 1.12                                                                             -- -- --                11 5   -- --  --                                                                              --                                                                              -- --  --   1.85 1.7 3.22                                                                             2.2                                                                              -- -- -- -- -- --                12 21  2.92                                                                             0.72                                                                              0.7                                                                             1.1                                                                             17 515 416  0.79 0.66                                                                              2.73                                                                             0.89                                                                             0.98                                                                             1  -- 105                                                                              225                                                                              4.85              13 22  2.97                                                                             0.63                                                                              1 1.6                                                                             25 486 517  0.66 0.72                                                                              2.71                                                                             0.76                                                                             1.25                                                                             1.18                                                                             -- 122                                                                              410                                                                              419               14 7   2.92                                                                             0.96                                                                              --                                                                              1.5                                                                             24 613 220  0.98 1.02                                                                              1.85                                                                             0.95                                                                             1.96                                                                             1.01                                                                             2.06                                                                             228                                                                              141                                                                              195               __________________________________________________________________________

The structure of Example 1 was especially suitable for embankmentstabilisation (see below) and had excellent properties in respect ofbreak load per meter width and tensile deformation.

The structures of Examples 2, 3 and 4 are uniaxially stretchedstructures, but the orientation does not pass right through the zones20. In Examples 2, 3 and 4 the lengths of the part of the zone 20 whichhas not been stretched is 7, 10.5 and 2.5 mm respectively, these valuesbeing 1.56, 7 and 2.5 times the thickness of the material respectively.

In Example 7, the mid-point of the zone 27 was very slightly thickerthan the mid-point of the strands 25, but in substance the thicknesseswere equal.

In Example 11, there was a w:d ratio of less than unity, and it will benoted that although the stretch ratios were relatively low, the whole ofthe junction 26 was orientated.

FIGS. 19 to 20d are taken from the product of Example 12 and FIGS. 21 to21d are taken from the product of Example 13.

Non-Uniform Structures

The mesh structures of this invention need not be uniform along thewhole of their length, and specific non-uniformaties can be introducedfor specific purposes, for instance to produce a carrier bag.

In one example, a tubular structure is in the form of sections ofuni-axially (machine direction) orientated mesh (as in FIG. 2) separatedby pieces of unstretched plastics material which when the tubularstructure is cut into suitable lengths, from the tops, or tops andbottoms, of carrier bags.

Uses

The uniaxially stretched structures can be used for instance forsunshades, crop shading, snow fencing, windbreaks, cladding material,anti-dazzle screens, insect screening or ground retaining orstabilising.

The biaxially stretched structures can be used for instance for stockfencing, horticultural use, civil engineering use, olive harvesting andreinforcement between laminated sheets.

Retaining or Stabilising Particulate Material

Structures generally in accordance with the invention can be used forretaining or stabilising particulate material of any suitable form, suchas soil, earth, sand, clay or gravel, and in any suitable location, suchas on the side of a cutting or embankment, beneath a road surface,runway surface or railway track, beneath a building or beneath a quay;it is considered that the structure can be specially suitable forpreventing retaining walls being forced out of position by the pressureof particulate material behind them. Retaining is a specific instance ofstabilising.

The preferred structure for retaining or stabilising is the uniaxiallystretched structure, though biaxially stretched structure could be used.

The mesh structure will normally be placed roughly parallel to thesurface of the particulate material, eg. horizontally below a roadway orinclined if near an embankment or cutting surface. The mesh structure ispreferably substantially rectilinear in section normal to its "plane,"at least in the section taken parallel to the orientated strands whichwill normally be parallel to the line of expected tension on the meshstructure. This enables the tensile strength of the mesh structure to befully exploited.

The mesh structure can have practical utility without specific fixedretention, but it is preferably fixed to at least one substantiallyrigid member. There may be just a single member for instance runningalong one edge of the mesh structure, or there may be two spaced,parallel members, for instance along opposite edges of the meshstructure, or there may be a number of parallel members spaced atintervals. The member(s) can be substantially normal to the orientatedstrands referred to above. The or each substantially rigid member ispreferably made of a cast material into which the mesh structure hasbeen incorporated before setting, a suitable material being concrete,but alternatively the mesh structure could be fixed in other ways to oneor more pre-cast members or to for instance one or more steel plates.The member eg. along one edge of the mesh structure could be a retainingwall.

In FIG. 12 a mesh structure 51 has had two opposite edge zones cast intosubstantially rigid concrete members or beams 52. The mesh structure hasparallel, orientated strands 53 and parallel bars 54, the mesh structurebeing a uniaxially stretched structure as described for instance withreference to FIG. 2 and Example 1. It will be seen that the bars 54 areincorporated in the beams 52 and during the moulding of the beams 52,the concrete is vibrated so that it passes in between the strands 53 andlocks firmly around the bars 54.

FIG. 13 shows the application of the structure of FIG. 12, to prevent aretaining wall 55 being forced out of position by the pressure of earth56. There are a number of parallel layers of the mesh structure 51,spaced one above the other, buried in the earth 56, the end beam 52 ofeach layer being incorporated in the retaining wall 55. It will be seenthat the beams 52 of one layer are positioned directly above the beams52 of the next layer. The expected tension on the mesh structure 51 isin the direction of the strands 53, and each layer is rectilinear in thesection of FIG. 13. The mesh structure itself has good slip resistanceproperties with respect to the earth 56, but the beams 52 (other thanthose in the wall 55) increase the slip resistance so that the meshstructures 52 act as a tie to prevent the retaining wall 55 being forcedout of its vertical position.

In FIG. 14, spaced layers of a uniaxially stretched structure 61, asdescribed for instance with reference to FIG. 2 and Example 1, areburied in an earth embankment.

Further Uses of the Invention

FIG. 15 illustrates a composite structure provided by erecting supportmembers in the form of fencing posts 71 and supporting a mesh structure72 thereby in a conventional manner. If the mesh structure 72 is toserve as for instance a highway anti-dazzle fence, it will be auniaxially-stretched structure in accordance with FIG. 2 with thestrands 17 extending vertically and the bars extending horizontally. Ifit is to serve for instance as stock fencing, it will be abiaxially-stretched structure in accordance with FIG. 3 with the strandsextending horizontally and vertically.

FIG. 16 illustrates a composite structure provided by erecting supportmembers in the form of posts 73 on a sea or river bed 74. The high waterlevel is at 75. A biaxially-stretched structure 76 in accordance withFIG. 3 is supported by the posts 73 in a conventional manner to serve asa fish trap or other submerged enclosure.

FIG. 17 illustrates a different form of submerged enclosure, namely acage or "pocket" for raising oysters. The sides 77 of the cage can actas the support members for the adjacent sides and top 78 and the bottomacts as a support member for the sides. The sides 77 and top 78, and thebottom if desired, are formed of a biaxially-stretched mesh structure inaccordance with FIG. 3 and are secured together in a conventionalmanner.

FIG. 18 illustrates the reinforcement of two films 79 by laminatingbetween them a biaxially-stretched mesh structure 80 in accordance withFIG. 3 which has on either side a layer of a low-melting plasticsmaterial, as referred to above. The structure 80 is heated byconventional means (not shown) before reaching laminating rolls 81, butnot to such a temperature that the main part of the structuredeorientates.

FIG. 19 illustrates a container 82 in the form of a sack or bag foragricultural or horticultural produce. The container 82 is formed oftubular biaxially-stretched mesh structure in accordance with FIG. 3,closed at the top and bottom by conventional means.

I claim:
 1. A method of producing a plastics material mesh structure,comprising:providing a substantially uniplanar plastics startingmaterial having a thickness of not less than 0.75 mm and defining apattern of holes or depressions whose centres lie on a notional,substantially square or rectangular grid of rows or columns; andstretching the starting material in a direction substantially parallelto the columns to stretch, into orientated strands, zones betweenadjacent holes or depressions of each row, the strands beinginterconnected by a set of substantially parallel bars generally atright angles to the strands; the stretching being effected to such anextent that a notional point which, on the starting material, lay on thenotional straight line which is parallel to the rows and is tangentialto respective holes or depressions, moves into the corresponding strandso that, on the mesh structure, the point is substantially spaced fromthe corresponding notional straight line which is parallel to the rowsof mesh openings and is tangential to respective mesh openings untilmolecular orientation of the plastics starting material is effected inat least a portion of the zone of each bar lying between the notionalstraight lines drawn tangentially to adjacent rows of holes ordepressions; the mid-point of the zone of the bar which interconnectsthe ends of aligned strands being substantially thicker than themid-point of either of the strands interconnected by the zone.
 2. Themethod of claim 1, wherein during the stretching operation a notionalpoint which on the starting sheet lay on the notional straight linewhich is parallel to the rows and is tangential to respective holes ordepressions, moved into the corresponding strand, and on the meshstructure is spaced from the corresponding notional straight line whichis parallel to the rows of mesh openings and is tangential to respectivemesh openings, by a distance which is not substantially less than 25% ofthe thickness of the mid-point of the respective strand.
 3. The methodof claim 2, wherein after the stretching operation the orientationpasses right through each notional junction zone, which zone is thenotional zone defined by the intersection of the notional parallel-sidedzones which lie between and are tangential to two columns or rows,respectively, of holes or depressions, from one strand to the alignedstrand on the other side of the notional junction zone.
 4. The method ofclaim 1, wherein the thickness of the starting material is at least 1mm.
 5. The method of claim 1, wherein the distance between adjacentholes or depressions in the same row is not substantially greater thanthe thickness of the starting material at the same point.
 6. The methodof claim 1, wherein the distance between holes or depressions ofadjacent columns in the starting material divided by the thickness ofthe starting material is 6.35 or less.
 7. A method of producing aplastics material mesh structure, comprising:providing a substantiallyuniplanar plastics starting material having a thickness of not less than0.75 mm and having a pattern of holes or depressions whose centres lieon a notional substantially square or rectangular grid of rows andcolumns; and stretching the starting material in a directionsubstantially parallel to the columns and also stretching in a directionsubstantially parallel to the rows, to form a generally square orrectangular grid of orientated strands interconnected by orientatedjunctions each of which has a minimum thickness not less than 75% of thethickness of the mid-point of any of the strands passing into thejunction, has a maximum thickness substantially greater than its minimumthickness and substantially greater than the thickness of the mid-pointof any of the strands passing into the junction, and has a central zonewhich is thicker than orientated zones on two opposite sides thereof,the edge zones of the crotches which extend between respective adjacentpairs of strands passing into the junction being orientated in thedirection running around the respective crotch.
 8. The method of claim7, wherein said central zone is long-shaped and aligned with two of thestrands passing into the junction.
 9. The method of claim 7, wherein thethickness of the mid-point of the central zone is substantially equal tothat of the mid-points of the strands with which the central zone isaligned.
 10. The method of claim 7, wherein the central zone extendsinto respective ends thereof, which latter zones are thicker than thecentral zone and are adjacent the ends of two respective alignedstrands.
 11. The method of claim 7, wherein, as measured in the plane ofthe mesh structure, the maximum dimension of each junction issubstantially aligned with two of the strands and is significantlygreater than the minimum dimension, which is aligned with the other twostrands, the sides of the junction merging very gradually with the sidesof the strands aligned with the maximum dimension and merging moreabruptly with the sides of the other strands.
 12. The method of claim 7,wherein the starting material is stretched in a direction substantiallyparallel to the columns and is subsequently stretched in a directionsubstantially parallel to the rows while allowing the structure toshorten in the first direction.
 13. The method of claim 7, wherein thestarting material is stretched in a direction substantially parallel tothe columns to stretch, into orientated strands, the zones betweenadjacent holes or depressions of each row, the stretching being effectedto such an extent that the orientation passes at least nearly rightthrough each notional junction zone, which zone is the notional zonedefined by the intersection of the notional parallel-sided zones whichlie between and are tangential to two columns or rows, respectively, ofholes or depressions, that material originally in the notional junctionzone is stretched out and forms the end portions of the strands, mergingsmoothly with the remainder of the respective strands, and that themid-point of each notional junction zone is substantially thicker thanthe mid-point of either of the strands passing into the junction, thestructure so formed being subsequently stretched in a directionsubstantially parallel to the rows.
 14. The method of claim 7, whereinthe distance between holes or depressions of adjacent columns in thestarting material divided by the thickness of the starting material is4.23 or less.
 15. The method of claim 7, wherein the structure isstretched until its final area on cooling divided by the area of thestarting material is 13 or more.
 16. The method of claim 7, wherein thestarting material is stretched in a first direction substantiallyparallel to the columns to form orientated strands extending in saidfirst direction until the orientation extends into notionalparallel-sided zones of the original starting material which are definedbetween notional lines extending in a second direction substantiallyparallel to the rows and tangential to the holes or depressions of therows on either side of said notional parallel-sided zones, but does notpass right through said notional parallel-sided zones, the startingmaterial being subsequently stretched in said second direction.
 17. Themethod of claim 7, wherein the starting material is stretched in a firstdirection substantially parallel to the columns and is subsequentlystretched in a second direction substantially parallel to the rows whilerestraining the structure in said first direction, thereby producing astructure in which the maximum thickness of each said junction is insaid central zone thereof, said central zone being in the form of a lumpwhich is surrounded by thinner, orientated zones with which the limpmerges.
 18. The method of claim 7, wherein the starting material isstretched in a first direction substantially parallel to the columns andis subsequently stretched in a second direction substantially parallelto the rows while the structure is not allowed substantially to shortenin said first direction.
 19. The method of claim 17 or 18, wherein thestarting material is stretched in said first direction while thestructure is restrained in said second direction.
 20. The method ofclaim 17 or 18, wherein the starting material is stretched in said firstdirection to form orientated strands extending in said first directionuntil the orientation extends into notional parallel-sided zones of theoriginal starting material which are defined between notional linesextending in said second direction and tangential to the holes ordepressions of the rows on either side of said notional parallel-sidedzones, but does not extend into said notional parallel-sided zones by adistance greater than about 30% of the width of said notionalparallel-sided zones.
 21. A method of producing a plastic material meshstructure, comprising:providing a substantially uniplanar plasticsstarting material having a thickness of not less than 0.75 mm and havinga pattern of holes or depressions whose centres lie on a notionalsubstantially square or rectangular grid of rows and columns; andstretching the starting material in a direction substantially parallelto the columns and also stetching in a direction substantially parallelto the rows, to form a generally square or rectangular grid oforientated strands interconnected by orientated junctions, thestretching being effected until the edge zones of the crotches whichextend between respective adjacent pairs of strands passing into thejunction are orientated in the direction running around the respectivecrotch, the the stretching being terminated while each junction has aminimum thickness not less than 75% of the thickness of the mid-point ofany of the strands passing into the junction, has a maximum thicknesssubstantially greater than its minimum thickness and substantiallygreater than the thickness of the mid-point of any of the strandspassing into the junction, and has a central zone which is thicker thanorientated zones on two opposite sides thereof.